1. Field of the Invention
This invention is directed to apparatus which operates cooperating rotors to break, rip and shear material to a size capable of passing through a perforated grate screen that is caused to move or oscillate so that substantially all areas of the grate screen are presented to the cooperating rotors.
2. Description of the Prior Art
Shredding apparatus for various items of waste material have been available in which parallel shafts have been provided with interleaved cutting or shearing elements. The shafts have been driven in opposite directions such that the interleaved elements operate to reduce the waste material in a shearing action, much like the action of scissors. It has been proposed also to provide the shredding apparatus with screen means at the outlet side of the shafts and interleaved elements to gauge the size of the reduced material. Furthermore, it has been proposed in prior apparatus to rotate the shafts at slow speeds and either in synchronism or at different speeds.
Shredding apparatus of the type in which parallel counter-rotating shafts, with shredding elements are employed, is exemplified by U.S. Pat. Nos. Panning et al 3,502,276 of Mar. 24, 1970; Brewer 3,578,252 of May 11, 1971; Rossler 3,662,964 of May 16, 1972; Schweigert et al 3,664,592 of May 23, 1952; Goldhammer 3,860,180 of Jan. 14, 1975; Cunningham et al 3,868,062 of Feb. 25, 1975; Baikoff 3,991,944 of Nov. 16, 1976; and Culbertson et al 4,034,918 of Jul. 12, 1977. The patents of Rossler and of Schweigert et al disclose fixed grates at the discharge side of comminuting apparatus.
Shredding apparatus disclosed in the prior art has a common problem with the accumulation of material that is partially reduced because of inactive or dead space left on the surface of the grate beneath the cutter discs on the rotor shafts, such dead spaces are located beneath the rotors that are interleaved on the respective shafts. The accumulation of material in the dead spaces is difficult to reduce except when other material entering the apparatus happens to crowd the material into the orbit of the rotors. Even reversible shredding apparatus has substantially the same difficulty.